Preventing accidental injury and infection from used hypodermic needles has attracted considerable interest in recent years, however previous proposals to produce syringes which retract the needle into the barrel of the syringe have required inconvenient locking or disengagement steps to release the needle from its injecting position so as to allow it to be withdrawn into the barrel.
Of interest in U.S. Pat. No. 4,026,287 which describes an arrangement in which the plunger has a screw fitting on its forward face. When the plunger reaches its fully depressed position, it is rotated to allow the screw fitting on the plunger to engage a corresponding thread on a forward portion of the syringe barrel in which the needle is seated. After securing the needle in this fashion, the plunger is withdrawn thus severing a weakening between the barrel and the forward needle portion to allow retraction of the needle. The requirement to screw engage the plunger and needle necessitates additional hand movements while the potentially dangerous needle is still exposed and further, severing the forward needle portion from the barrel requires significant force.
Some previous proposals, for instance as shown in U.S. Pat. Nos. 4,692,156, 4,507,117 and 4,804,370 or PCT Publication Nos. WO89/04681 and WO89/09075 have utilized a snap connection between a fitting on the forward end of the plunger and a formation associated with the needle or its mounting. These arrangements allow engagement of the plunger and needle (mount) without a distinct locking or screwing step. However, in these arrangements the needle (mount) is adhered or force fit into sealing engagement with the barrel and force must be applied during retraction of the plunger to break this adhesive or other connection between the needle (mount) and the barrel. This is disadvantageous as not only does this rearward force need to be applied while the needle is exposed, but the engagement between the plunger and the needle (mount) must be robust enough to prevent unlocking of the needle (mount) during application of this force.
The syringes depicted in U.S. Pat. No. 4,838,869 and Australian Patent Application No. 28334/89 provide a more active release of the needle or its mount from the remainder of the barrel. In these syringes, the plunger, in its fully depressed position engages the forward wall of the barrel, adjacent the needle, thus breaking the adhesive or seal between the needle or its mount and the barrel allowing the needle (mount) to be retracted without applying a significant rearward force. These arrangements, however, are not amenable to easy production techniques and furthermore do not appear to be well adapted for syringes in which a removable needle is located on a needle seat of the needle mount.
The arrangement depicted in PCT Publication No. WO89/00435 also uses the downward motion of the plunger to disengage the needle from the barrel, in this case allowing it to be spring biased into a recess in the plunger. As with the syringes discussed in the immediately preceding paragraph, the arrangement shown in WO89/00435 is not amenable to easy mass production and furthermore is incapable of being used with a replaceable needle as is required, for instance, when a syringe is filled from an ampoule using a first needle but injected into a patient using a fresh sterile needle. A further disadvantage of the WO89/00435 syringe is the comparatively fragile nature of the seating of the needle in the barrel before retraction which is likely to prematurely retract the needle if the syringe is inadvertently inserted into unyielding material such as callous or muscle.
It is an object of the present invention to ameliorate some of the shortcomings evident in previous proposals to provide a syringe in which the needle mount is automatically engaged by the plunger for safe retraction into the barrel.